loop: remember whether sysfs_create_group() was done

syzbot is hitting WARN() triggered by memory allocation fault
injection [1] because loop module is calling sysfs_remove_group()
when sysfs_create_group() failed.
Fix this by remembering whether sysfs_create_group() succeeded.

[1] https://syzkaller.appspot.com/bug?id=3f86c0edf75c86d2633aeb9dd69eccc70bc7e90b

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+9f03168400f56df89dbc6f1751f4458fe739ff29@syzkaller.appspotmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Renamed sysfs_ready -> sysfs_inited.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -809,16 +809,17 @@ static struct attribute_group loop_attribute_group = {
 	.attrs= loop_attrs,
 };
 
-static int loop_sysfs_init(struct loop_device *lo)
+static void loop_sysfs_init(struct loop_device *lo)
 {
-	return sysfs_create_group(&disk_to_dev(lo->lo_disk)->kobj,
-				  &loop_attribute_group);
+	lo->sysfs_inited = !sysfs_create_group(&disk_to_dev(lo->lo_disk)->kobj,
+						&loop_attribute_group);
 }
 
 static void loop_sysfs_exit(struct loop_device *lo)
 {
-	sysfs_remove_group(&disk_to_dev(lo->lo_disk)->kobj,
-			   &loop_attribute_group);
+	if (lo->sysfs_inited)
+		sysfs_remove_group(&disk_to_dev(lo->lo_disk)->kobj,
+				   &loop_attribute_group);
 }
 
 static void loop_config_discard(struct loop_device *lo)

drivers/block/loop.h

@@ -58,6 +58,7 @@ struct loop_device {
 	struct kthread_worker	worker;
 	struct task_struct	*worker_task;
 	bool			use_dio;
+	bool			sysfs_inited;
 
 	struct request_queue	*lo_queue;
 	struct blk_mq_tag_set	tag_set;